Although initially believed to be used by the archangels as the weapon to destroy the knights, Cain later revealed that it was he who killed the Knights, except for Abaddon.

When Cain realized that his brother Abel was talking to Lucifer, and not God, Cain offered an exchange where his younger brother would go to Heaven and Cain would take his place in Hell. Lucifer agreed, on the condition that Cain himself would send his brother to Heaven. Cain then fashioned a weapon out of the jawbone of an animal and used it to kill his brother.

Lucifer gave Cain a mark, which acted as the blade's source of power. This made the First Blade powerful enough to kill anything that Lucifer could kill, including the Knights of Hell. However, without the mark, the blade would simply be a bone.While when killing a normal demon, the Blade causes a glow similar to the Demon-killing knife or an angel blade, when killing a Knight of Hell it causes a massive burst of orange/red light to emerge from the Knight's vessel, similar to Cain's smiting ability, and the glow of an angels vessel when he dies.

Weaknesses

It appears that if the bearer of the Mark dies, he or she will automatically come back as a demon without being tortured in Hell.